1. Field of the Invention
Embodiments of the present invention relate to an emission control system for reducing nitrogen oxide emissions from the exhaust of an internal combustion engine.
2. Background Art
The exhaust from a combustion engine exhaust often contains a variety of waste components including unburned hydrocarbon (HC), carbon monoxide (CO), particulate matter (PM), nitric oxide (NO), and nitrogen dioxide (NO2), with the NO and NO2 collectively referred to as the oxides of nitrogen or NOx.
Selective Catalytic Reduction (SCR) with ammonia (NH3) can be used for NOx control on lean-burn gasoline engines and diesel engines. A reductant such as ammonia (NH3) is often supplied from an external source, and the SCR catalyst uses base metals to promote the reaction between the NH3 and the NOx in the exhaust to produce N2 and H2O under lean conditions. In certain other instances, a liquid solution of urea [CO(NH2)2] can be injected instead of NH3 for vehicle applications. When injected into an exhaust, the urea breaks down into molecules of NH3.
An important feature of SCR catalysts is that the SCR catalysts can store NH3 in the washcoat at temperatures below 400° C. This stored NH3 can be particularly useful during transient driving conditions, where the exhaust flow rate and/or the NOx concentration can change rapidly. If the injected flux of NH3 is less than the instantaneous NOx flux, the stored NH3 may be used to supplement the injected NH3. Conversely, if the injected flux of the NH3 is more than the instantaneous NOx flux, the excess NH3 can be stored on the SCR catalyst.
However, at temperatures above 400° C., the NH3 storage capacity of the SCR catalyst decreases substantially. At these higher temperatures, the NH3 has to be carefully injected to match or closely correspond to the NOx flux in order to maximize the NOx conversion and minimize the NH3 slip. This can be a significant challenge during transient driving conditions. If too little NH3 is injected, the NOx conversion will be decreased, and if too much NH3 is injected, the NH3 slip will increase uncontrollably.
Moreover, certain SCR catalysts may oxidize excess NH3 to NOx at certain higher temperatures which results in additional NOx emission.
As such, it would be desirable to provide an emission control system wherein the supply of SCR reductant such as NH3 is effectively controlled for an improved NOx reduction and/or optimized reduction in the NH3 slip.